During the combustion process, fuel is oxidized, and hydrogen (H) and carbon (C) combine with air. Various chemical compounds are formed including carbon dioxide (CO2), water (H2O), carbon monoxide (CO), nitrogen oxides (NOx), unburned hydrocarbons (HC), sulfur oxides (SOx), and other compounds. Automobile exhaust systems include an exhaust treatment device, such as a three-way catalytic converter, that reduces CO, HC and NOx in the exhaust gas. The exhaust treatment device must be heated to a sufficient temperature for optimum performance.
Diesel engines have improved thermal efficiency as compared to gasoline engines. As a result, the exhaust temperature of a diesel engine is lower than that of a gasoline engine. The exhaust provides heat that is used to warm the exhaust treatment device. Because the exhaust temperatures of diesel engines are lower than that of gasoline engines, the diesel exhaust does not consistently heat the exhaust treatment device to desired operating temperatures.